Item Summary

Hawaii Institute of Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii

Citation:

Hawaii Institute of Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii. June 1992. Silica Recovery and Control in Hawaiian Geothermal Fluids: Final Report. Honolulu, Hawaii: Hawaii Institute of Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii.

Abstract:

U.S. DOE Grant No. DE-FG07-88ID12741. A series of experiments was performed to investigate methods of controlling silica in waste geothermal brines produced at the HGP-A Generator Facility. Laboratory testing has shown that the rate of polymerization of silica in the geothermal fluids is highly pH dependent and increases rapidly as the brine pH is increased. At brine pH values in excess of 8. 5 the suspension of silica polymers flocculated and rapidly precipitated a gelatinous silica mass. Optimum flocculation and precipitation rates were achieved at pH values in the range of 10.5 to 11.5. The addition of transition metal salts to the geothermal fluids similarly increased the rate of polymerization as well as the degree of precipitation of the silica polymer from suspension. A series of experiments performed on the recovered silica solids demonstrated that methanol extraction of the water in the gels followed by critical point drying yielded surface areas in excess of 300 m2/g and that treatment of the dried solids with 2 N HCl removed most of the adsorbed impurities in the recovered product. A series of experiments tested the response of the waste brines to mixing with steam condensate and non-condensable gases. The results demonstrated that the addition of condensate and .NCG greatly increased the stability of the silica in the geothermal brines. They also indicated that the process could reduce the potential for plugging of reinjection wells receiving waste geothermal fluids from commercial geothermal facilities in Hawaii. Conceptual designs were proposed to apply the gas re-combination approach to the disposal of geothermal waste fluids having a range of chemical compositions. Finally, these designs were applied to the geothermal fluid compositions found at Cerro Prieto, Ahuachapan, and Salton Sea. Our analysis indicates that they could, under appropriate circumstances, be used at Cerro Prieto and Ahuachapan but not at the Salton Sea geothermal field.